Silicon nitride, silicon carbide and mixtures thereof have generated considerable interest as ceramic materials. They have high thermal and oxidative stability and, if maximum theoretical density can be achieved, are among the hardest materials that can be made at atmospheric pressure. Other advantageous properties include low electrical conductivity, low coefficient of thermal expansion, excellent thermal shock and creep resistance, high strength at elevated temperatures and corrosion resistance.
Commercial interest in silicon nitride and silicon nitride/silicon carbide materials is high. There have been three major routes for the preparation of silicon nitride:
1. high temperature reaction of gaseous nitrogen with elemental silicon ("nitridation"); PA0 2. gas phase reaction of ammonia with a chlorosilane (SiCl.sub.4, HSiCl.sub.3, H.sub.2 SiCl.sub.2) at higher temperatures; and PA0 3. reaction of ammonia with SiCl.sub.4 or HSiCl.sub.3 in solution, followed by pyrolysis of the insoluble ammonolysis product after removal of ammonium chloride. PA0 1. formation into complex shapes and subsequent pyrolysis to give a ceramic material of the same shape; PA0 2. spinning into continuous fibers whose subsequent pyrolysis yields ceramic fibers; PA0 3. as a matrix material for carbon or ceramic fibers, or as a binder for ceramic powders (with subsequent pyrolysis to form a ceramic body); PA0 4. oxidation-resistant coatings on otherwise oxidizable materials (such as pyrolytic graphite) - after the polymer coating is made, it can be pyrolyzed to give the resistant ceramic coating; PA0 5. infiltration of porous ceramic bodies such as ones obtained from reaction-sintered silicon nitride by the polymer itself (if liquid) or by a solution of the polymer, with subsequent pyrolysis to form a ceramic, resulting in better strength, oxidation resistance, etc. of the body; and PA0 6. formation of thin films of the ceramic material for electronics applications.
Recently, Seyferth et al., J. Amer. Ceram. Soc. 66 pp. C-13 to C-14 (1983) described the formation of a soluble silazane polymer by reaction of ammonia with H.sub.2 SiCl.sub.2 in a suitable solvent. This polymer can be pyrolyzed in a nitrogen atmoshphere to produce Si.sub.3 N.sub.4.
There is currently great interest in preceramic polymer materials such as described by Seyferth et al., supra, the pyrolysis of which yield Si.sub.3 N.sub.4, SiC or Si.sub.3 N.sub.y /SiC materials. Applications for such polymers include, among others:
For instance, Penn et al., J. Appl. Polymer Sci. 27 pp. 3751-61 (1982) describe the preparation of silicon carbide-silicon nitride fibers from a polycarbosilazane precursor. Tris(N-methylamino)methylsilane monomer was formed by reaction of monomethylamine and methyltrichlorosilane in dry petroleum ether and a polycarbosilazane resin was formed by passing the monomer over glass raschig rings at 520.degree. C. The brittle polymer was soluble in methylene chloride and chloroform, etc. This product was spun into fibers, crosslinked in air and then pyrolyzed to give ceramic fibers.
Other polymer precursors for forming silicon carbide and silicon nitride ceramics have been described in U.S. Pat. Nos. 3,108,985; 3,853,567; 3,892,583; 4,310,651 and 4,312,970. These linear or crosslinked polymers and processes for producing ceramic materials have generally been found to be deficient in one or more ways.
It would be hightly desirable to have a polymer precursor for Si.sub.3 N.sub.4 /SiC ceramic materials that is formed from readily available and relatively cheap starting materials in high yield; that is liquid or, if solid, is soluble in organic solvents; that is stable at room temperature for prolonged periods; that is relatively stable to hydrolysis by atmospheric moisture; and that can provide a high yield of ceramic material upon pyrolysis.